An injection device, for example an injection pen, can be driven electromechanically by using a motor, which drives a transmission to force a liquid substance out of a container.
The container may be a cartridge, for example for a medicament. The cartridge may have a cylindrical body made of glass, plastics, and/or the like. A first end of the cartridge may be closed by a septum which may be pierced by a needle. A second end of the cartridge may be closed by a plunger which is movable inside the cylindrical body of the cartridge to force out the liquid substance or medicament through the needle in the pierced septum.
With direct current motors, there is a direct relationship between the current consumption of the motor and the output torque. That is, the electrical power drawn by the motor is proportional to the mechanical power provided by the motor to a load. The rotation speed of the motor thus depends on the load and varies with changes in the load.
For injection devices, a stepper motor may be used instead of a direct current motor, since the rotation of the stepper motor can be controlled precisely. A stepper motor can be controlled to rotate by a selected number of steps regardless of the mechanical power taken off by the load. Since there is a direct relation between the amount of movement of a transmission driven by a motor and the amount of rotation of the motor, it is possible to drive a transmission in a sequence of steps and thereby to discharge a desired dose of the liquid substance.
A microcontroller may be employed for computing the number of steps required for achieving the desired dose and for generating corresponding control signals for the stepper motor.
The available torque of a stepper motor depends primarily on the provided current, while the rotation frequency only depends on the control signals provided to the motor. If less mechanical power is taken off at the motor shaft than could be provided by the motor, the excess power is converted into heat. In contrast to a direct current motor, there is thus no direct relation between the consumed electrical power and the provided mechanical power.
Generally, an electronic circuit is used to control a sequence in which coils of the stepper motor are energized to obtain a desired rotation, and the current is kept constant irrespective of the operating voltage and the rotation frequency. This means that usually more electrical power is used than required for the provided mechanical power.
As a result, the energy efficiency is not optimal. In particular in the case of battery driven devices, however, high energy efficiency is desirable to obtain a high durability of the batteries.
Document EP 0 481 376 A2 describes an apparatus for controlling a stepping motor that is used for driving a chemical pump. The apparatus includes a random access memory for storing a minimum current value required for driving the chemical pump at each of divisional intervals of one cycle of the chemical pump, two photo interrupters for sensing an angular location of the stepping motor corresponding to each of the intervals of the chemical pump, and a central processing unit connected to the two photo interrupters for controlling the stepping motor in accordance with the angular location of the stepping motor sensed by the two photo interrupters, the central processing unit being so arranged that it uses the minimum current value required for driving the chemical pump at an angular location following the angular location sensed by the two photo interrupters.
Document WO 2005/093533 A1 suggests modifying an electrical current value driving an infusion pump stepper motor in response to distal pressure information and other information. The distal pressure is a backpressure resisting the forward pressure generated by the infusion pump in applying medication intravenously. With a common type of pump, a plastic tube leading from a bag or bottle on a drip stand to the intravenous needle passes through a special gate in which it is occluded between a row of ‘fingers’ which are moved by a cam mechanism to squeeze the closed point forward.